1. Field of the Invention
The present invention relates to a bidirectional controlling device, and more particularly, to a bidirectional controlling device for increasing resistance of elements on voltage stress.
2. Description of the Prior Art
In specifications of a conventional thin-film transistor liquid crystal display (TFT LCD) having a medium or small size, there are requirements of bidirectional scanning. While low temperature poly-silicon (LTPS) or amorphous silicon (a-Si) are utilized for fabricating thin-film transistors (TFTs), and when a gate driving circuit is formed on a glass substrate, for meeting the requirements of bidirectional scanning, a bidirectional circuit has to be added in the gate driving circuit for controlling a direction of scanning signals.
FIG. 1 illustrates a bidirectional controlling device conventionally used in a gate driving circuit. A bidirectional controlling device 100 includes a first gate input terminal 109, a second gate input terminal 110, a first thin film transistor (TFT) 101, a second TFT 102, a third TFT 103, a fourth TFT 104, a first input terminal 105, a second input terminal 106, a first output terminal 107, and a second output terminal 108. Note that the TFTs in the bidirectional controlling device 100 are switches.
In the bidirectional controlling device 100, the first gate input terminal 109 and the second gate input terminal 110 outputs two DC signals having mutually-opposite polarities. Assume the first gate input terminal 109 provides an input signal XBi, and assume the second gate input terminal 110 provides an input signal Bi. When the input signal Bi from the second gate input terminal 110 receives a high voltage level, the input signal XBi from the first gate input terminal 109 receives a low voltage level. At this time, since an input signal of the first input terminal 105 is transmitted to the first output terminal 107 through the first TFT 101, and since an input signal of the second input terminal 106 is transmitted to the second output terminal 108 through the fourth TFT 104, an operation of the bidirectional controlling device 100 may be called forward scanning. On the contrary, when the input signal Bi from the second gate input terminal 110 receives a low voltage level, the input signal XBi from the first gate input terminal 109 receives a high voltage level. At this time, since the input signal of the first input terminal 105 is transmitted to the second output terminal 108 through the second TFT 102, and since the input signal of the second input terminal 106 is transmitted to the first output terminal 107 through the third TFT 103, the operation of the bidirectional controlling device 100 may be called reverse scanning.
In the bidirectional controlling device 100, bidirectional controlling signals for controlling the direction of scanning, i.e., the input signal XBi provided by the first gate input terminal 109 and the input signal Bi provided by the second gate input terminal 110, are DC signals, and are directly inputted to gates of TFTs of the bidirectional controlling device 100. Under conventional circumstances, when the bidirectional controlling device 100 is implemented with a-Si, shifts of threshold voltages of a-Si-based elements interfere significantly with a control mechanism of the bidirectional controlling device 100. Due to the DC voltage stress on the a-Si-based elements, the threshold voltage shifts are easily generated on the a-Si-based elements. In other words, the threshold voltages exceed predicted degrees so that conducting currents are significantly reduced.